1. Field of the Invention
The present invention relates generally to a nitride compound semiconductor element.
2. Related Background Art
GaN, which is one of compound semiconductors of Groups III–V containing nitrogen, has a large band gap of 3.4 eV and is direct gap semiconductor. So, there has been grate interest in nitride compound semiconductor as a material of a short-wavelength light emitting element. Also, there has been grate interest in nitride compound semiconductor as a high output electronic element material that needs a high insulating strength because of its large band gap. In the past, it was considered that the crystal growth of GaN was difficult because there were no good substrates that coincided in lattice constant and thermal expansion coefficient with GaN. However, a method of growing GaN crystals on a sapphire substrate having a lattice mismatch of 15% with GaN has then been developed, using a low-temperature buffer layer. According to this method, a thin amorphous or polycrystal AlN or GaN film is grown as a buffer layer on the sapphire substrate at a low temperature of about 600° C. to modify the effect of the lattice mismatch and then an device structure section of GaN is grown on the buffer layer at a high temperature of about 1000° C. This method provides a layered structure of nitride semiconductors containing GaN formed on the sapphire substrate to thereby realize a nitride compound semiconductor element such as a blue-light emitting diode and/or a royal purple-light emitting laser.
With the conventional nitride compound semiconductor elements, however, the sapphire substrate is prone to be warped to thereby lower productivity and/or yield. It has been considered that this warp cannot be avoided as long as the sapphire substrate is used that is different scores of % in coefficient of thermal expansion from the nitride semiconductor.
More particularly, with the conventional GaN crystal growth, the sapphire substrate was warped at its midline so as to be convex toward the nitride semiconductor because large residual stresses were produced in the GaN. It has been considered in the conventional technical commonsense that the large residual stresses are produced necessarily due to a difference in coefficient of thermal expansion between the sapphire substrate and the nitride semiconductor layer and can not be avoided. The production of the residual stresses in GaN is prone to produce threading dislocations and/or cracks in the GaN. Especially, when the substrate is large, the threading dislocations and/or cracks become remarkable. Thus, it has been considered in the past that crystal growth for a nitride compound semiconductor element on a large substrate is extremely difficult and that a deterioration in the productivity and/or yield cannot be avoided compared to semiconductor elements of other materials.
The inventor considers that no satisfactory characteristics are obtained due to the above-mentioned large residual stresses in the prior art nitride compound semiconductor elements. The inventor, however, considers that nitride compound semiconductor elements having improved characteristics compared to the prior art will be obtained by reducing the large residual stresses, other unnecessary stresses and crystal defects such as the threading dislocations.